The present invention is directed generally to angle positioning platforms. More particularly, the present invention is directed to angle positioning platforms or gimbals for positioning payload such as a spacecraft engine around first and second axes.
Positioning platforms are commonly used to move an object on first and second axes. Typically, the first and second axes are perpendicular to one another. Conventional angle positioning platforms have a number of limitations. When positioning some objects, a large range of rotational motion is necessary. For example, in some spacecraft an engine must be moved from a stowed position during launch, to a working position during transfer orbit, and finally to a new working position once on station. In the working positions the engine may require bidirectional angular adjustments. Movement from the stowed position to the transfer orbit working position, or from this position to the on station working position may require large angular rotation (e.g., 25 to 35 degrees). In the working positions the engines periodically need small angular adjustments (e.g., 0 to 5 degrees).
Some conventional angle positioning platforms lack a rotational range wide enough to carry a payload (e.g., engine) from the stowed position to a working position (e.g., transfer orbit position) and from one working position to another (e.g., on station position). These applications generally require two devices: a deployment actuator for large angle motion from the stowed position to the working position and a positioning device for the small angular adjustment required in the working position. Such a system usually cannot accommodate two working positions.
Another shortcoming of some conventional angle positioners, particularly those positioners having two output gearboxes, is the large amount of space and weight required for such positioners.
Another shortcoming of some conventional angle positioning platforms is the vulnerability to damage during heavy loads such as may be experienced in spacecraft during launch. Angle positioning platforms typically include precise gears and bearing assemblies. During launch, the gears and bearing assemblies undergo stresses that may impair the later performance of the positioning platform. These stresses experienced by the gears and bearing assemblies are greater when the positioning platform is supporting a heavy object. Thus, many conventional positioning platforms are not capable of supporting large loads such as heavy engines (e.g., ion engines). In some conventional platforms, heavy components must be used in order to have enough strength to withstand the stresses encountered during launch. Yet those heavy components are stronger than necessary to perform during on station use.
Thus, there is a need for 2-axis positioning platforms that have high angular ranges and that are also compact, lightweight, and capable of withstanding high stresses, such as stresses encountered during launch. There is also a need for a 2-axis positioning platform that reduces the load experienced by delicate components of the platform during launch.
Some drive mechanisms for motor-driven positioning platforms include wormgear assemblies or gearboxes to convert rotation of a motor into rotation of a member about a particular axis. Wormgear assemblies typically have a worm shaft having a worm, a wormgear with teeth in mating engagement with the worm, and a main shaft connected to the wormgear. Backlash between the wormgear and the worm shaft is minimized in some conventional wormgear assemblies by torsion springs disposed around the main shaft. However, the torque provided by torsion springs varies within manufacturing tolerances and, consequently, requires some conventional assemblies to use larger, heavier springs than would otherwise be necessary in order to tolerate the variation and provide torque in the required range. Alternatively, more expensive torsion springs may be required which are made to tighter tolerances.
Further, torsion springs often come into contact with the main shaft, creating friction that effectively lessens the torque provided by the springs and possibly generating debris. To compensate for the decrease in torque from friction, heavier, stronger springs are sometimes required. Thus, there is a need for an antibacklash mechanism that reduces friction between the springs and the main shaft.
In some typical devices having wormgear assemblies, a member is connected to the main shaft of the wormgear assembly for rotation with the main shaft. In some of these devices, rotation of the worm shaft is stopped, when desired, by impeding movement of the member. The impeding of the member stops the rotation of the main shaft which, in turn, stops the rotation of the wormgear. In these conventional wormgear assemblies, the worm stops rotating when the friction and other forces between the worm and the wormgear teeth prevent further worm rotation.
When the worm is stopped as described above, a number of undesirable consequences arise. The components in the drive train between the wormgear and the member attached to the main shaft are placed under stress. Also, the worm becomes wedged against the wormgear teeth. This wedging of the worm creates wear on the wormgear assembly and may, if the rotational force of the worm is high enough, permanently deform either the worm or the wormgear teeth. Furthermore, the worm might become wedged so forcefully against the wormgear teeth that the worm becomes jammed in the wormgear and cannot be rotated in the opposite direction. Such a circumstance would prevent subsequent repositioning of the platform.
Thus, there is a need for a wormgear assembly having an antibacklash system without the aforementioned problems. There is also a need for a mechanism for stopping rotation of the wormgear without the aforementioned problems.
When an engine is mounted on a positioning platform, the engine may undergo a large angular rotation. The fuel line has an end connected to the engine and thus the fuel line must be able to withstand and not inhibit the rotation of the engine. Ion engines are particularly sensitive to impurities and water vapor. Conventional flexible materials, such as plastic, often are not suitable for use with ion engines because of evolving impurities and the tendency of conventional flexible materials to trap water vapor.
Metallic fuel lines may be used to avoid the problems of water vapor and other impurities. However, solid metallic fuel lines are limited in angular flexibility and are susceptible to fatigue failure, therefore limiting their life in cyclic flexing applications. Corrugated or bellows tubing has much greater elastic angular flexibility and may be used effectively about a single axis. In a two axis application, a single bellows tube would experience axial torsion in addition to bending. The bellows tube does not have elastic flexibility in torsion and therefore resists this motion and will prematurely fail due to fatigue. A separate bellows tube could be used for each axis, however, this would require additional plumbing and mounting hardware, thus requiring greater space and weight.
Also, some conventional fuel lines are made from materials that require the fuel lines to be relatively long in order to have sufficient flexibility. Such long fuel lines require loops and guides which add weight to the spacecraft. Thus, there is a need for a fuel line assembly that is composed of materials suitable for use with spacecraft ion engines, that is compact, that can accommodate the rotation of the engine on two axes and that is not susceptible to premature fatigue failure.